Public and private sectors have interest in renewable fuels and chemicals from biomass to replace fossil fuel from finite oil reserves. Woody and herbaceous biomasses are making a comeback after over a century of domination by coal, oil, and natural gas. This interest is driven in part by the carbon-neutral footprint of the biomass. One of the proposed routes is to convert cellulose and hemicellulose to fermentable sugars as a feedstock for bio-based chemicals.
The Green Power+® technology has been developed by American Process, Inc. This technology extracts hemicelluloses from a biomass feedstock supply and converts only those hemicelluloses into sugars which are then fermented, such as to cellulosic ethanol. Green Power+ technology is a two-step process to produce sugars from hemicelluloses. An initial steam or hot-water extraction pulls out hemicelluloses, and the remainder of the biomass is suitable for combustion in a boiler or for pelletization, or for cellulose hydrolysis, or other uses. The extracted solution is then hydrolyzed with a mild acid or enzyme treatment to hydrolyze oligomers into fermentable monomers.
All biomass is composed of cellulose fibers which are bound to hemicelluloses and glued together by lignin to form a matrix, which is naturally deconstruction-resistant. Only after loss of viability, the biomass starts to rot by fungi, which secrete enzymes to deconstruct the biomass matrix. These enzymes are categorized as cellulases, hemicellulases, or laccases depending of targeted function. Recently, these enzymes are harvested for commercial use. The drawbacks of enzymes include slow activity and sensitivity to environmental conditions. This typically leads to long retention times at moderate temperatures, which increases the risk of contamination by bacteria or wild yeast.
The retention time and enzyme dosage can be reduced, if the structural matrix accessibility is improved. The possible pretreatment methods include mechanical, thermal, and chemical methods. In particular, hydrothermal methods can be beneficial for hemicellulose removal. Most of the hemicellulose is soluble in hot pressurized water or steam. After sufficient pretreatment, the cellulose fibers are released from the matrix or can be physically blown apart by a pressure release. This deconstruction allows enzymes an access to the cellulose.
The access requires also a means of introducing the enzyme so that it is effectively distributed into the biomass. This may be difficult, when the amount of enzyme solution is small compared to biomass. Dilute solution facilitates the enzyme transfer into the fibrous material. Low-consistency solutions of up to 8% allow carrier fluid, usually water, to surround fibers in the liquid. Pumping and mixing at this concentration present little challenge. However, the volume of over 90% water makes saccharification process equipment large, and dilutes the subsequent sugar concentration.
Higher temperature can be attempted for faster kinetics and thus smaller equipment. Cellulases and hemicellulases generally have activity at temperatures from 30° C. to 70° C. At the high end of the temperature range, however, the enzyme denatures for permanent loss of activity.
Generally, enzymatic hydrolysis for pretreated biomass requires optimization of enzyme feed as one of the most expensive process components. Some methods use enzyme recycle in a batch mode by squeezing out the enzymes and introducing to fresh biomass, which requires energy and wastes enzyme activity when enzymes bind with high concentrations of non-cellulose fiber active sites (such as lignin). Batch recycling only works for some enzymes that can bind to active sites quickly before being squeezed or removed.
Enzymatic hydrolysis of pretreated biomass remains as a significant commercial challenge for the biorefinery industry. Improved methods of using enzymes, and process and systems incorporating such methods for enzymatic hydrolysis, are desired.